1. Field of the Invention
The present invention relates to the calibration of angular displacement of an object about a rotary axis, such as a rotary table.
2. Description of Related Art
It is known to calibrate a rotary table using an angular interferometer together with an indexing table which is mounted co-axially with the rotary table, and which may rotate relative to the rotary table in equi-angular and accurately known indexable increments which are mechanically defined by the construction of the table. Angular displacement of the rotary table may be measured by the angular interferometer by detecting rotation of a pair of retroreflectors mounted on the indexing table (when the indexing table rotates with the rotary table). The angular interferometer may detect rotary displacement over a range of about .+-.15.degree. from a datum position (which is the position at which the plane of the two retroreflectors lies exactly perpendicular to the incident of beams of the angular interferometer).
The calibration operation using this apparatus is performed by coupling the indexing table to the rotary table (and therefore coupling the retroreflectors of the angular interferometer to the rotary table); rotating the rotary table to a predetermined target angular displacement; decoupling the indexing table and counter-rotating the indexing table by a number of indexing positions which is approximately equal to the target angular displacement of the rotary table; and interpolating with the interferometer any net angular displacement of the retroreflectors from the datum position. The counter-angular displacement of the indexing table (which is accurately known from the indexed construction of the indexing table), is then summed with the net angular displacement measured by the interferometer and compared with the predetermined target angular displacement of the rotary table. A difference between these two values represents the error in the rotary table at the predetermined target angular displacement.
One problem with such a table occurs when the target angle of the rotary table exceeds the maximum angular displacement which the interferometer may continuously measure (typically .+-.15.degree.). One proposed solution to this problem has been to prevent rotation of the indexing table relative to the datum during rotation of the rotary table; i.e. the indexing and rotary tables are uncoupled during rotation of the rotary table to its target angle and the indexing table is mechanically retained at a position which lies within a few degrees of the datum, so that the indexing table effectively simultaneously counter-rotates relative to the rotary table. This method is inconvenient for the user.